1. Field of the Invention
This invention relates to a control device for Venetian blinds and its control method, particularly to one having safety with a rope not exposed and handling convenience of a rope exposed.
2. Description of the Prior Art
Conventional control device for Venetian blinds are generally classified into two kinds, one with a rope exposed and the other with a rope not exposed.
The rope-exposed control device for Venetian blinds includes an upper rail, a lower rail, a window covering positioned between the upper rail and the lower rail, such as cloth member of a winding blind, a plurality of strips of a Venetian blind, and a moving up-and-down device for combining the window covering between the upper and the lower rail. The moving up-and-down device generally is composed of a ratchet fixed on the upper rail, and plural strips of the window covering placed between the upper and the lower rail and a pull rope arranged between the ratchet and the window covering. The rope has a portion extending out of the ratchet and is exposed for a user to pull or loosen for controlling a position of the window covering moved together with the lower rail. But this kind of the partially exposed rope is a no-end loop-shaped one, which may cause potential danger of hurt or suffocation with the neck of a child caught by the rope in pulling it as a play.
As for the non-exposed one, it includes an upper rail, a lower rail, a window covering positioned between the upper and the lower rail, and a moving up-and-down device combining the window covering between the upper and the lower rail. The moving up-and-down device has plural springs fixed on the upper rail, and a position rope pulling the window covering between the upper and the lower rail. Then the position of the window covering is controlled by the control device to be altered as needed. Maybe other moving up-and-down devices can be used such as utilizing the tension of the position rope. Generally speaking, the non-exposed rope device is not so convenient as the exposed rope control device in handling. Moreover, there is also a disadvantage that the spring may lose its resiliency after a period of use, or the position rope may lose its tension sooner or later.